1. Field of the Invention
The present invention relates to a power transmission coupling and a power transmission apparatus.
2. Description of Related Arts
As a power steering apparatus for a motor vehicle to which a steering assisting force is given from an electric motor, there is known apparatus comprising a first steering shaft connected to the steering wheels, and a second steering shaft connected to the first steering shaft through a torsion bar and also connected to the steering mechanism. This apparatus is arranged in the following manner. A torque sensor detects the amount of relative displacement of the first and second steering shafts in the rotation direction, and based on the result of detection by the torque sensor, the rotation of the electric motor is transmitted to the second steering shaft through a speed reduction mechanism. Accordingly, the operation of the steering mechanism according to the rotation of the steering wheels, is assisted by the rotation of the electric motor, thus lightening the steering burden upon the vehicle driver.
As the speed reduction mechanism, there is used a mechanism comprising a worm and a worm wheel. The worm shaft having the worm mounted thereon, and the second steering shaft having the worm wheel mounted thereon, are supported at their axial ends by bearings, thus preventing the worm shaft and the second steering shaft from being moved in the radial and axial directions.
Provision is made such that when the worm shaft and the worm wheel are assembled, no backlash is produced therebetween within the range of machining precision. However, when the teeth of the worm and the worm wheel are increased in wear after assembling, backlash is produced, contributing to noise.
In view of the foregoing, it is proposed, as in Japanese Patent Laid-Open Publication 2000-43739 for example, to make provision such that the worm shaft can be biased toward the worm wheel, thus successfully eliminating the backlash by biasing the worm shaft in the biasing direction by biasing means.
In the arrangement in the Publication above-mentioned, however, power transmission between the motor and the worm shaft is made with the use of spline fitting. The spline fitting hardly allows a radial displacement of the worm shaft. It is therefore practically difficult to combine the arrangement in the Publication with the backlash adjusting mechanism above-mentioned. Accordingly, the backlash adjusting mechanism above-mentioned cannot be adopted, thus failing to assure stillness.
To assure stillness, there is proposed to form a power transmission coupling by a synthetic resin. However, the synthetic resin may be insufficient in strength. In particular, when the power transmission coupling is used for the electric pump for power steering of a motor vehicle, the power transmission coupling is raised to a high temperature. Thus, the resin tends to be lowered in strength. Even though it is intended to use spline fitting for the power transmission coupling, it is difficult to form thin and strong spline teeth by a synthetic resin. Accordingly, the proposal of forming a power transmission coupling by a synthetic resin in order to assure stillness, can be realized with difficulty as far as spline fitting is used, and therefore can assure no stillness.
The problem that no stillness can be assured, is present not only in the drive system of a motor vehicle, but also in other drive system.
It is an object of the present invention to provide a power transmission coupling and a power transmission apparatus, both of which are excellent in stillness.
To achieve the object above-mentioned, the present invention provides, as a preferred aspect, a power transmission coupling comprising a power transmission member having first and second ends, which are respectively connected to ends of corresponding shafts through corresponding first and second sliding pairs, which are different in sliding direction from each other, the power transmission member above-mentioned being made of a fiber-reinforced synthetic resin of a polymer alloy comprising polyamide MXD6 and polyphenylene ether, to which glass fibers are being blended.
The use of a synthetic resin as the material of the power transmission member securely provides stillness. The polymer alloy is originally high in material strength, and its material strength is hardly lowered at high temperature. When such a polymer alloy is further reinforced with fibers, the high strength can be maintained even at high temperature. Further, the polymer alloy is reinforced with glass fibers. This is generally more economical as compared with reinforcement with other fibers than glass fibers, e.g., carbon fibers.
The polyamide MXD6 contained in the base resin is a crystalline thermoplastic polymer obtained from methaxylene diamine (MXDA) and an adipic acid, and has an aromatic ring as the backbone chain. Examples of the polyamide MXD6 include xe2x80x9cRENYxe2x80x9d manufactured by Mitsubishi Engineering Plastics Co., Ltd. The polymer alloy is a multi-component polymer obtained by mixing or chemically bonding polymers containing a plurality of components.
The reason why the polymer alloy comprising MXD6 and polyphenylene ether is used as the base resin, is because the advantages of both MXD6 and polyphenylene ether can profitably be utilized. More specifically, the polymer alloy can obtain the advantages of the crystalline MXD6 excellent in chemical resistance, toughness and strength and the advantages of amorphous PPE such as low water absorption, no glass transformation and high rigidity. Thus, the disadvantages of both MXD6 and PPE can be made up for.
Preferably, the concentration of the glass fibers above-mentioned is in the range of 30 to 50% by weight. A concentration less than 30% by weight causes the strength to be insufficient, while a concentration exceeding 50% by weight makes molding difficult.
The present invention provides, as a preferred aspect, a power transmission apparatus for transmitting an electric power from an electric motor, comprising: a motor shaft disposed at the electric motor; a worm shaft on which a worm is disposed; a power transmission coupling comprising a power transmission member which connects the motor shaft and the worm shaft to each other in a power transmittable manner; a driven shaft on which disposed is a worm wheel engaged with the worm; a mechanism for adjusting backlash between the worm wheel and the worm in association with the worm shaft; and first and second sliding pairs which respectively connect first and second ends of the power transmission member to respective ends of the motor shaft and the worm shaft, the rotation of the motor shaft being transmitted to the driven shaft through the power transmission coupling, the worm shaft, the worm and the worm wheel, the first and second sliding pairs being different in sliding direction from each other, the first and second sliding pairs allowing the end of the worm shaft to be radially displaced at the time of backlash adjustment.
According to the embodiment above-mentioned, at the time of backlash adjustment, the end of the worm shaft is displaced in the radial direction of the power transmission coupling, and such a displacement can be allowed by the first and second sliding pairs. When the power transmission coupling having the arrangement above-mentioned is used, it becomes practically possible to incorporate, in the power steering apparatus, the mechanism for adjusting backlash in the worm mechanism as a speed reduction mechanism. This results in achievement of stillness.